This invention relates to films containing polybenzazole polymers and processes for making them.
Polybenzazole polymers are typically synthesized by polymerization in non-oxidizing dehydrating acid solutions to form viscous dopes containing the acid and the polymer. Uniaxially oriented film may be synthesized from dope by extruding onto a rotating drum that draws the film in the machine direction and rotates it down into a water bath to coagulate. Biaxially oriented films are synthesized by extruding the dope as a tube, which is blown or forced over a mandrel to impart some biaxial orientation, and is then immersed in water to coagulate it. Examples of such processes are described in: Chenevey, U.S. Pat. No. 4,487,735 (Dec. 11, 1984); Lusignea et al., U.S. Pat. No. 4,871,595 (Oct. 3, 1989); Chenevey, U.S. Pat. No. 4,898,924 (Feb. 6, 1990); Harvey et al., U.S. Pat. No. 4,939,235 (Jul. 3, 1990); Harvey et al., U.S. Pat. No. 4,963,428 (Oct. 6, 1990); Lusignea et al., U.S. Pat. No. 4,966,806 (Oct. 30, 1990); and Fujiwara, Japanese Kokai 63(1988)-74612 (published Apr. 5, 1988), which are incorporated herein by reference.
Improvements are needed in the processes for making polybenzazole films. Films made by the previously described processes tend to have very inconsistent gauge thickness. The biaxial orientation is less than complete, so that films still tend to have better properties in the machine direction and poorer properties in the transverse direction. Moreover, the overall properties of the films are not very high. A new process for synthesizing biaxially oriented polybenzazole films is needed.